The prior method for producing a polycarbonate in industry includes an transesterification method by which a dihydroxy compound and a carbonic acid diester compound are used as the starting materials and these are subjected to transesterification to obtain a polycarbonate.
This transesterification method is roughly classified into a molten transesterification method and a solid phase polymerization method, and hence, these are explained in this order below.
(1) Molten Transesterification Method (JP-A-61-62,522, JP-A-2-153,925)
The case where bisphenol A is used as the dihydroxy compound and diphenyl carbonate is used as the carbonic acid diester compound is taken as an example and explained. This molten transesterification method is a method for producing a polycarbonate which comprises melting bisphenol A and diphenyl carbonate and subjecting them to transesterification reaction in the molten state to eliminate the phenol.
In this molten transesterification method, however, the melting point and melt viscosity of the polycarbonate polymer increase with the progress of polymerization reaction, and hence, a reaction temperature near 300.degree. C. is finally required and a surface renewal by stirring as much as possible and a high vacuum of not more than 1 mm Hg are required for the deaeration-removal of phenol produced as a by-product. Accordingly, in this molten transesterification method, an apparatus enabling high temperature, high vacuum and strong stirring is indispensable and hence the equipment cost increases necessarily.
Moreover, since in this molten transesterification method, the polycarbonate polymer produced is exposed in the vicinity of the decomposition temperature of the polycarbonate for a long period of time, the coloring of the polycarbonate and the deterioration of the polycarbonate due to decomposition reaction and side reactions such as cross-linking, branching and the like are caused with a high possibility.
Accordingly, in the current technique relating to a high viscosity fluid, there is a limit in the molecular weight of a polycarbonate which can be stably produced in industry by the molten transesterification method, and it is considered to be about 20,000 in terms of a viscosity average molecular weight.
(2) Solid Phase Polymerization Method (JP-A-63-223,035, JP-A-1-158,033)
Similarly, the case where bisphenol A is used as the dihydroxy compound and diphenyl carbonate is used as the carbonic acid diester compound is taken as an example and explained. This solid phase polymerization method is a method for producing a high molecular weight polycarbonate which comprises subjecting bisphenol A and diphenyl carbonate to transesterification reaction in the molten state to produce an oligomer, thereafter crystallizing the oligomer by a solvent-treatment, a heat-treatment or the like and subjecting the crystallized oligomer to solid phase polymerization.
This solid phase polymerization method enables the reaction to be conducted at a temperature not higher than the melting point of the polycarbonate (e.g., not more than 240.degree. C.), so that the polycarbonate obtained does not have the problem of coloring polycarbonate which has been recognized in the above molten transesterification method and is superior in quality to the polycarbonate obtained by the molten transesterification method.
However, in this solid phase polymerization method, at least three steps including the oligomerization step, the crystallization step and the solid phase polymerization step are essential, and not only is the production operation complicated, but also a high equipment cost and a high production cost are unavoidable. The solid phase polymerization for obtaining a high molecular weight polycarbonate from the oligomer can be conducted at a low temperature; however, the transesterification reaction rate depends upon the temperature, and the higher the temperature, the higher the reaction rate. Therefore, when the solid phase polymerization is effected at a low temperature, a long reaction time is required. For example, in order to obtain a high molecular weight polycarbonate from the oligomer by the solid phase polymerization process, a reaction time 1.5 to 10 times that in the molten transesterification method is required.
Accordingly, the object of this invention is to provide a process for producing a polycarbonate which has overcome the disadvantages of the conventional method for producing a polycarbonate, especially the solid phase polymerization method and can produce a high quality, high molecular weight polycarbonate in steps as few as possible with a good efficiency in a short time at a low cost.